Electrochemical cells commonly contain a negative electrode (anode) and a positive electrode (cathode), an electrolyte permeable separator therebetween and an electrolyte in contact with both of the electrodes. Electrolytes can be aqueous-based or non-aqueous organic solvent-based liquid electrolytes or polymeric electrolytes. There are two basic types of electrochemical cells, a primary (nonrechargeable) and a secondary (rechargeable) cell. A primary electrochemical cell is discharged to exhaustion only once. A secondary electrochemical cell, however, is rechargeable and thus can be discharged and recharged multiple times.
Primary (non-rechargeable) lithium cells have an anode comprising lithium and a cathode comprising manganese dioxide, and electrolyte comprising a lithium salt such as lithium trifluoromethane sulfonate (LiCF3SO3) dissolved in a mixtures of nonaqueous solvents. These lithium cells (Li/MnO2 cells) are commonly in the form of button (coin shaped) cells, prismatic or polyhedral cells (wherein one or more of the housing surfaces are flat, typically of cuboid, namely, rectangular parallelepiped shape) or cylindrical cells, e.g. ⅔ A cell having about ⅔ the height of conventional AA alkaline cells. (The ⅔ A cell has an IEC designation “CR17335” and has a diameter of about 15 mm and height of about 32 mm). The Li/MnO2 cells have a voltage of about 3.0 volts which is twice that of conventional Zn/MnO2 alkaline cells and also have a higher energy density (watt-hours per cubic centimeter of cell volume) than that of alkaline cells. (Alkaline cells as referenced herein shall be understood to be conventional commercial alkaline cells having an anode comprising zinc, a cathode comprising manganese dioxide, and an electrolyte comprising aqueous potassium hydroxide.) Therefore, Li/MnO2 cells can be used in compact electronic equipment, especially photographic cameras, which require operation at higher voltage and at higher power demand than individual alkaline cells.
Primary lithium electrochemical cells typically employ an anode of lithium metal or lithium alloy, preferably a lithium-aluminum alloy; a cathode containing an electrochemically active material consisting of a transition metal oxide or chalcogenide, preferably manganese dioxide; and an electrolyte containing a chemically stable lithium salt dissolved in an organic solvent or a mixture of organic solvents. (The term “anode active material” or “cathode active material” as used herein shall be understood to mean material in the anode or cathode, respectively, which undergoes useful electrochemical reaction during cell discharge, contributing to the cell's capacity and voltage.)
The lithium anode is preferably formed from a sheet or foil of lithium metal or lithium alloy without any substrate or lithium metal deposited or coated on a metallic substrate such as copper or other metals. A lithium primary cell referenced hereinafter as having an anode comprising “lithium” shall be understood to mean an anode of lithium metal or a lithium alloy. If a lithium-aluminum alloy is employed, the aluminum is present in a very small amount, typically less than about 1 wt % of the alloy. The addition of aluminum primarily serves to improve the low temperature performance of the lithium anode in lithium primary cells.
Manganese dioxides suitable for use in lithium primary cells include both chemically produced manganese dioxide known as “chemical manganese dioxide” or “CMD” and electrochemically produced manganese dioxide known as “electrolytic manganese dioxide” or “EMD”. CMD can be produced economically and in high purity, for example, by the methods described by Welsh et al. in U.S. Pat. No. 2,956,860. However, CMD typically does not exhibit energy or power densities in lithium cells comparable to those of EMD. Typically, EMD is manufactured commercially by the direct electrolysis of a bath containing manganese sulfate dissolved in a sulfuric acid solution. Processes for the manufacture of EMD and representative properties are described in “Batteries”, edited by Karl V. Kordesch, Marcel Dekker, Inc., New York, Vol. 1, 1974, pp.433-488. Manganese dioxide produced by electrodeposition typically is a high purity, high density, “gamma(γ)-MnO2” phase, which has a complex crystal structure containing irregular intergrowths of a “ramsdellite”-type MnO2 phase and a smaller portion of a beta(β)- or “pyrolusite”-type MnO2 phase as described by deWolfe (Acta Crystallographica, 12, 1959, pp.341-345). The gamma(γ)-MnO2 structure is discussed in more detail by Burns and Burns (e.g., in “Structural Relationships Between the Manganese (IV) Oxides”, Manganese Dioxide Symposium, 1, The Electrochemical Society, Cleveland, 1975, pp. 306-327).
Electrochemical manganese dioxide (EMD) is the preferred manganese dioxide for use in primary lithium cells. However, before it can be used, it must be heat-treated to remove residual water. The term “residual water”, as used herein includes surface-adsorbed water, noncrystalline water (i.e., water physisorbed or occluded in pores), as well as lattice water. Heat-treatment of EMD prior to its use in lithium cells is well known and has been described by Ikeda et al. (e.g., in “Manganese Dioxide as Cathodes for Lithium Batteries”, Manganese Dioxide Symposium, Vol. 1, The Electrochemical Society, Cleveland, 1975, pp. 384-401).
EMD suitable for use in primary lithium cells can be heat-treated at temperatures between about 200 and 350° C. as taught by Ikeda et al. in U.S. Pat. No. 4,133,856. This reference also discloses that it is preferable to heat-treat the EMD in two steps. The first step is performed at temperatures up to about 250° C. in order to drive off surface and non-crystalline water. The EMD is heated in a second step to a temperature between about 250 and 350° C. to remove the lattice water. This two-step heat-treatment process improves the discharge performance of primary lithium cells, primarily because surface, non-crystalline, and lattice water are all removed. An undesirable consequence of this heat-treatment process is that EMD having the γ-MnO2-type structure, is gradually converted to EMD having a gamma/beta (γ/β)-MnO2-type structure. The term “gamma/beta-MnO2” as used in the art reflects the fact (as described by Ikeda et al.) that a significant portion of the γ-MnO2 (specifically, the ramsdellite-type MnO2 phase) is converted to β-MnO2 phase during heat-treatment. At least about 30 percent by weight and typically between about 60 and 90 percent by weight of the ramsdellite-type MnO2 phase is converted to β-MnO2 during conventional heat treatment of γ-MnO2 as taught, for example, in U.S. Pat. No. 4,921,689. The resulting γ/β-MnO2 phase is less electrochemically active than an EMD in which the γ-MnO2 phase contains a higher fraction of ramsdellite-type MnO2 relative to β-MnO2. Thackeray et al. have disclosed in U.S. Pat. No. 5,658,693 that cathodes containing such β-MnO2-enriched phases exhibit less capacity for lithium uptake during discharge in lithium cells.
One consequence of the electrodeposition process used to prepare EMD is that the formed EMD typically contains “residual surface acidity” from the sulfuric acid of the electrolytic bath. This “residual surface acidity” must be neutralized, for example, with basic aqueous solution, before the EMD can be used in cathodes for primary lithium cells. Suitable aqueous bases include: sodium hydroxide, ammonium hydroxide (i.e., aqueous ammonia), calcium hydroxide, magnesium hydroxide, potassium hydroxide, lithium hydroxide, and combinations thereof. Typically, commercial EMD is neutralized with a strong base such as sodium hydroxide because it is highly effective and economical.
An undesirable consequence of the acid neutralization process is that alkali metal cations can be introduced into ion-exchangeable sites on the surface of the EMD particles. For example, when sodium hydroxide is used for acid neutralization, sodium cations can be trapped in the surface sites. This is especially undesirable for EMD used in cathodes of primary lithium cells because during cell discharge the sodium cations can be released into the electrolyte, deposit onto the lithium anode, and degrade the lithium passivating layer. Further, the deposited sodium cations can be reduced to sodium metal, react with the organic electrolyte solvents, and generate gas, thereby substantially decreasing the storage life of the cells.
A process for converting commercial grade EMD that has been neutralized with sodium hydroxide to the lithium neutralized form is disclosed by Capparella et al. in U.S. Pat. No. 5,698,176 and related Divisional U.S. Pat. No. 5,863,675. The disclosed process includes the steps of: (a) mixing sodium hydroxide neutralized EMD with an aqueous acid solution to exchange the sodium cations with hydrogen ions and produce an intermediate with reduced sodium content; (b) treating the intermediate with lithium hydroxide or another basic lithium salt to exchange the hydrogen ions with lithium cations; (c) heat-treating the lithium ion-exchanged EMD at a temperature of at least about 350° C. to remove residual water. However, Capparella et al. disclose that “contacting particulate EMD with high pH lithium hydroxide solution may also serve to introduce lithium ions into the crystal lattice of the MnO2, thereby altering the crystal structure into a form which is not useful as a cathode active material”. Further, Capparella et al. specifically teach against treatment of an aqueous suspension of EMD with lithium hydroxide to a final pH greater than 7.5 since such treatment was disclosed to destroy EMD particle integrity and produce sub-micron size MnO2 particles that were difficult to process.
In place of manganese dioxide a lithiated manganese dioxide can be used as the cathode active material for primary lithium cells. A method for preparing a lithiated manganese dioxide and its use in primary lithium cells as cathode active material in primary lithium cells is described in U.S. Pat. No. 6,190,800. The lithiated manganese dioxide recited in this reference is a heat treated lithiated manganese dioxide product having the formula LiyMnO2−δ, wherein 0.075≦y≦0.175 and 0.01≦δ≦0.06, and a predominantly gamma(γ)-MnO2-type crystal structure.
Thus, as evidenced by the cited prior art, the methods used to prepare active cathode materials comprising manganese dioxide or lithiated manganese dioxide suitable for cathodes in a primary lithium cells require additional refinement in order to substantially improve performance of the lithium cells incorporating such active cathode materials.